The present invention relates generally to charge image storage tubes having a direct viewing storage target including a storage dielectric layer of phosphor material capable of bistable storage of a charge image and which emits a light image corresponding to such charge image.
More particularly, the invention relates to an improved storage dielectric of phosphor particles coated with smaller particles of high secondary electron emissive material bonded only to the surface of such phosphor particles and distributed uniformly throughout a layer of the dielectric, and to a method of manufacture of such a storage dielectric. The method of manufacture comprises coating phosphor particles with a secondary emissive-forming substance, and then heating the coated phosphor particles to produce the secondary emissive material and, simultaneously, bond such secondary emissive material to the phosphor particles.
The improved storage dielectric produced by the method of the present invention is especially useful in direct viewing bistable storage tubes of the type employed in certain storage type cathode-ray oscilloscopes, but may also be used in radar and sonar apparatus, as well as in other display devices such as those employed as part of data transmission systems.
Previous bistable storage tubes employing phosphor storage dielectrics have had a slow "writing speed" (the maximum deflection velocity at which a stored charge can be produced by a writing beam, due to the relatively low secondary electron emission of the phosphor material. In order to increase writing speed it has been proposed to add higher secondary electron emissive material, such as magnesium oxide, to the phosphor storage dielectric by loosely mixing particles of magnesium oxide with particles of phosphor material. While this does increase writing speed, unfortunately it has the disadvantage of greatly reducing the brightness of the light image emitted by the phosphor material due to the large amount of magnesium oxide required. As a result, prior art bistable storage tubes employing phosphor storage dielectrics either have high brightness, about 6 foot-lamberts, but relatively slow writing speed, approximately 25,000 centimeters per second, if the secondary emissive additive is omitted, or have a relatively low brightness, about 2 foot-lamberts, and a high writing speed, 100,000 centimeters per second, if such secondary emissive additive is used. Thus previously it was thought that both high brightness and fast writing speed were not possible when using a storage dielectric of phosphor material.
The method of the present invention solves the above-mentioned problem by providing the high secondary emissive material as a coating of smaller particles on the surface of the phosphor particles. In this way, the secondary additive material is used most efficiently, greatly reducing the amount needed to increase writing speed by the desired amount. By using a lesser amount of high secondary emissive material, the resulting storage dielectric produces a higher brightness light image, due in part to the greater percentage of phosphor material, and in part to lower attenuation of such light image by the secondary emissive material. In addition, by disposing secondary emissive material on only a portion of the surface of each phosphor particle, low velocity flood electrons are able to strike both the phosphor particle and its secondary emissive coating simultaneously. This makes more efficient use of the phosphor material since the secondary emissive material does not prevent electrons from striking the phosphor. In this regard, it should be noted that the low velocity flood electrons are accelerated through a potential difference of only about 200 to 500 volts, so tha they will not pass through a layer of secondary emissive material on a phosphor particle if the particle is completely masked by the material. This in fact occurs in prior storage dielectric employing a mixture of phosphor material and secondary emissive material.
The method of storage dielectric manufacture of the present invention forms a high secondary emissive material from a substance coated on the phosphor particles, so that the material becomes intimately bonded to the phosphor particles as it is formed. By contrast, in previous storage dielectrics the secondary emissive material is formed separately before it is mixed with the phosphor particles, and is not bonded to the phosphor particles.
The phosphor storage dielectric can be made sufficiently porous to enable collection of secondary electrons through the dielectric, and the addition of secondary emissive material produced by the method of the invention does not interfere with such collection because it is bonded to the phosphor particles and does not fill the pores between adjacent phosphor particles, unlike previous storage dielectrics in which the material is merely mixed with the phosphor. In addition, it has been found that the storage dielectric of the present invention has a longer useful lifetime than previous phosphor storage dielectrics.
Accordingly, one object of the present invention to provide improved bistable storage apparatus which is capable of fast writing speed in producing a stored charge image and which emits a light image of high brightness corresponding to the stored charge image.
Another object of the present invention is to provide an improved storage tube of simple and economical construction which employs a storage dielectric comprised of phosphor particles and high secondary emissive material coated on such phosphor particles.
Still another object of the present invention is to provide an improved storage dielectric of long, useful lifetime which is capable of bistable storage of a charge image produced thereon at a fast writing speed, and which emits a light image of high brightness corresponding to the stored charge image by employing phosphor particles coated with high secondary emissive material bonded to the particles.
A further object of the present invention is to provide an improved method of manufacture of a phosphor storage dielectric capable of bistable charge image storage by which a high secondary emissive material is formed from a substance previously coated on the phosphor particles so that the secondary emissive material formed is simultaneously bonded to the phosphor particles.
An additional object of the present invention is to provide an improved storage dielectric of phosphor particles coated by smaller particles of high secondary emissive material bonded to the phosphor particles to enable efficient use of a small amount of secondary emissive material to increase the writing speed of the storage dielectric without greatly decreasing the brightness of the light image emitted by the phosphor particles and, at the same time, to provide a longer useful lifetime for the storage dielectric.